Laminators are constantly being challenged to reduce or eliminate visual defects in laminated glass structures. Some defects can be directly attributed to glass quality but many are considered to be associated with the laminating process and more specifically with the commonly used polyvinyl butyral (PVB) interlayer. Defects can look like bubbles or pockets of air with elongated worm-like or dendritic shapes. Elongated worm-like and dendritic defects are often referred to as delamination. Some defects are visible immediately after autoclaving, but others develop hours or days after lamination. Laminators who use vacuum for de-airing tend to experience higher defect rates in warm weather.
Traditionally, delamination is viewed as the result of adhesive bond failure between the glass and the PVB interlayer. That is, the adhesive forces cannot withstand the stresses that are due to mismatches in the glass as well as gaps and pinches. A typical explanation for defects that are near a laminate""s edge, is that the PVB absorbs moisture from the environment, which lowers the adhesion level leading to defect formation. Therefore, it is rationalized that during warm and humid seasons, moisture is absorbed at a higher rate, and hence causes more defects.
In the laminating industry there is general agreement that gaps and pinches do lead to defects. In fact gaps of approximately 0.1 mm in height over a distance of 5 cm are suspects for causing defects. The load required to achieve a 0.1 mm gap or pinch can be calculated from mechanical considerations, and it is a surprisingly low, 1.0 N/cm for 2.1-mm thick glass. For this reason, adhesive forces cannot explain the formation of most defects.
If the adhesion level is primarily responsible for defects, then higher adhesion levels should be able to overcome more stress, and hence, would accommodate larger gaps and pinches without causing a defect. However, our findings have shown this not to be true.
With respect to moisture absorption, the adhesive interlayer absorbs moisture from the environment until equilibrium is reached. The equilibrium level depends on the relative humidity and may differ for different interlayers. The mechanism for moisture absorption is diffusion, which means that the concentration of the diffusant is highest at the phase boundary (i.e., at the laminate""s edge). A typical moisture profile of a PVB laminate exposed to 95% relative humidity at 40xc2x0 C. for one week shows that only interlayer within 3-4 mm from the edge has moisture higher than 1.5%, and the moisture level hardly changes about 8 mm in from the edge. Most of the observed defects occur about 3-12 mm away from the edge and some extend slightly farther inwards. Very few defects are open to the edge where the moisture level is highest and where one would expect to have the lowest level of adhesion. Worm-like defects have been observed to disappear spontaneously while windshields are stored in hot, humid environments.
It is possible to adjust the adhesivity of the PVB interlayer so that even when laminated at high moisture, the final adhesion is suitable for use in automobile windshields. However, laminates made this way would fail if they are installed into automobiles which are driven in or exposed to high ambient temperatures. Bubbles form readily at temperatures less than 100xc2x0 C. in laminates where the PVB interlayer has been equilibrated prior to lamination to a relative humidity higher than 50%. These laminates most likely would not pass the bake test or the boil test required by national and international standards (e.g., ANSI Z26, JIS R-3212, EC R-43, and others).
Another reason moisture intrusion does not explain many of the defects is that even in the absence of high moisture, adhesion at 30xc2x0 C. is only a fraction of what it is at room temperature. Increasing the adhesion between the glass and the adhesive interlayer at room temperature, therefore, would not help to eliminate defects [that] tend to occur at higher temperatures. Further, correlation between data from tests run at temperatures well below room temperature, such as the pummel test, and delaminations is at least questionable.
The typical approach in attempting to solve delamination problems has been to include various additives in the adhesive sheet to increase the strength of the adhesive bond between the sheet and the glass plate. While such approaches have been successful in changing the adhesive level, and to some extent reducing delaminations, increasing adhesion upsets the delicate adhesion balance required to provide acceptable laminated products. It is well known that an adhesion level that is too high can make the laminate fail like a piece of monolithic glass and unable to absorb much energy upon an impact, or if the adhesion is too low, on impact large glass shards fly from the structure. In each instance, changing the adhesion level renders the laminate unacceptable.
We have found that the presence of air plays a most significant role in defect formation in laminated glass. De-airing and edge seal must be as complete as possible in pre-pressed laminates before autoclaving in order to avoid defects. Poor de-airing has been linked to delaminations in windshields after they have been shipped from their fabrication sites. In some cases, delaminations have caused automobile assembly lines to halt production, and the automobile manufacturers to incur substantial financial losses. The issue for the laminator often is that it is difficult to determine whether de-airing in a laminate is complete since a windshield can appear clear due to the dissolution of air in the PVB interlayer under high pressure during autoclaving, the final step in the lamination process.
It has now been found that delayed delaminations can be avoided by using surprisingly small amounts of nucleating agents as indicators for excess air in windshields and other laminated structures.
In accordance with the present invention, a glass/adhesive sheet laminar structure comprising at least one layer of glass and a sheet of plasticized polyvinyl butyral (PVB) which is not susceptible to delayed delamination is provided wherein the sheet of plasticized polyvinyl butyral (PVB) is stabilized by using one or more nucleating agents. Such nucleating agents may be incorporated in the PVB, applied to the surface of the PVB sheeting or applied to the layer of glass which is to be adhered to the PVB sheeting. Like boiling chips, nucleating agents lower the energy barrier for phase transitions and encourage the formation of small bubbles. When sufficient amounts are provided, the bubbles would be small and isolated, the stress from supersaturated air in the PVB interlayer is released, the nuclei or tiny pockets of air which are entrapped in the sheeting and are prevented from coalescing to form worm-like delaminations. Other properties of the laminar structure, such as adhesion, need not be changed necessarily by the use of the nucleating agent. The PVB has blended therein an adhesion control agent to provide a preselected level of adhesion between the layer of glass and sheet of PVB. Selected amounts of a nucleating agent may be blended with the PVB. The nucleating agent may be applied to the surface of the PVB sheet or the surface of the glass which contact each other. The amounts are further selected so that the level of adhesion between the layer of glass and sheet of PVB is not significantly changed, i.e. not changed more than about 20% of the absolute value of the adhesion.
Generally, from about 0.0001 to 0.1 part by weight of the nucleating agent per 100 parts by weight of the PVB resin will allow small nuclei to form in the event that there is supersaturated air in the plasticized PVB interlayer without any substantial change in the level of adhesion. Compounds that are not readily soluble in or miscible with the PVB interlayer should be selected. Examples encompass inert solids such as colloidal silica, insoluble hydroxides of alkaline earth metals, humectants or astringents such as chlorohydrates of aluminum and zirconium, mildly incompatible compounds such as polyethylene glycol and polydimethylsiloxane, and solids which form in situ during the extrusion of PVB sheets or during lamination such as magnesium, calcium and zinc salts of n-heptanoic and other carboxylic acids of four carbons or more.